Here presented is a specific calculation method bassed on the priciples of geometric optics of the advanced Cassegrain Antenna. And another calculation method is given to determine the amplifude and phase of the three-mode Conical Horn of TE₁₁, TM₁₁ and TE₁₁ by applying the aperture，field method，and also the figures of its phase center. Finally，the authors propose some ideas for improving the efficiecy of the antenna.